Relative sizes of prokaryotic and eukaryotic cells

In lecture on Friday, we introduced the two basic classes of cellular organization: eukaryotes, which surround their genetic material (DNA) with a membrane to form a structure called the nucleus, and prokaryotes, which lack a membrane-bound nucleus. Although the two types of cells differ greatly in a number of ways, one way that they differ is in size. Prokaryotic cells are generally smaller than eukaryotic cells, and then we launched into a brief discussion of why this might be so. In this image from our textbook, we can see a human cheek cell (a eukaryote) that is covered with many bacteria (prokaryotes,) and it should be immediately apparent that the human cell greatly dwarfs the bacteria. This relationship generally holds true when comparing eukaryotic cells with prokaryotic cells. In our explanation of why this is the case, we used the concepts of surface area and volume of a cell to try and understand what happens when a cell gets bigger.

A small cell and a big cell, with nutrients moving to the center

In order to explain this in a different way, consider two balls constructed of a porous material such as a sponge. The balls are in every way identical in terms of composition, however one of them is ten times wider than the other, so that the first has a radius of 1 centimeter, and the second has a radius of 10 centimeters. To appreciate how much bigger the second one is, we could weigh them on a scale. Since they are composed of the same material, they have the same density, which I will say is equal to 1 gram per cubic centimeter. If we were to weigh them both, we would find that the smaller one weighs just about 4 grams, and the larger one weighs 1000 times more, or just over 4 kilograms. Now consider if we put each of these sponges into a pool of water; how quickly would each of them get completely soaked? The smaller one would become saturated much more quickly, as water can penetrate to the center more easily. The larger one takes longer to get soaked to the center. If we now think of these as cells, and the sponge material actually represents enzymes and cell parts that require nutrients for them to work, you can see that it will be harder to get sufficient nutrients all through the larger cell, as everything has to come through the outside surface in order to get to the center.

The larger cell has invaginations from the surface

In class, we said that eukaryotic cells solve this dilemma by having internal membrane compartments (organelles) that in effect make the surface area of the cell bigger relative to a small cell. In this version of the two cells, I have drawn the larger one so that the surface penetrates deep inside the cell. Now nutrients can get to the center much more quickly of the larger cell by diffusing into the invagination and entering the cell all along the length of the channel. The larger cell is able to take up nutrients (and get rid of metabolic wastes) much more quickly with these structures than if it lacked them. The complicated internal structure of eukaryotic cells has allowed them to attain quite remarkable sizes relative to prokaryotic cells, resulting in the very largest cells, the oocyte.

There are however, examples of prokaryoticcells that achieve extremely large sizes. They are able to accomplish this feat of giganticism by the same mechanism that eukaryotic cells do: they have internal membranes to effectively increase their relative surface area, allowing them to carry out a useful rate of growth.

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About ycpmicro

My name is David Singleton, and I am an Associate Professor of Microbiology at York College of Pennsylvania. My main course is BIO230, a course taken by allied-health students at YCP. Views on this site are my own.

Prokaryotes and Eukaryotes are very different and exciting cells to learn about, and the mechanical aspect of cells is one of my favorite topics in biology. I throughly enjoyed reading this post because it made thinking about the size relation and way they intake differently in a new perspective for me. I always knew that Eukaryotes were bigger in size but I never really thought about how it is harder for them to ingest nutrients due to their larger size. Its so interesting how such a small cell has evolved to greater sizes and create new organelles inside the cell to make up for the larger size, and help the cell acting like enzymes to speed up the intake process or nutirents.

I’m really glad you liked it. When I first applied for this position at YCP (they were advertising for a Microbiologist,) I had to admit when I was on campus that I was a “Cell Biologist” first, and a “Microbiologist” second; I find it fascinating to study how things work, and I hope to instill some of that into this class. We will introduce a topic called the Endosymbiont Hypothesis that works to explain how the complicated internal structures of eukaryotic cells arose from the association and internalization of prokaryotes into other cells.